1. Field of the Invention
This invention relates to a switching power supply device which is usable as a DC power source for various types of electric apparatus and implements. More particularly, the present invention a switching power source circuit arrangement which is so designed that power loss is reduced and thus efficiency is enhanced.
2. Description of the Prior Art
Referring to FIG. 1 of the accompanying drawings, a pulse-width controlled type switching power supply circuit is shown as an example of the prior-art switching power supply circuit arrangement, wherein a main switching transistor Q.sub.1 has its emitter connected to an input terminal 1 to which is applied a DC input voltage V.sub.i, the collector of the transistor Q.sub.1 being grounded through a primary winding N.sub.1 of a transformer T. Furthermore, the transistor Q.sub.1 has its base connected to a pulse width control circuit 2 through a base-current limiting resistor R.sub.1, and also to the input terminal 1 through a resistor R.sub.2 which serves to prevent collector cutoff current from being caused to flow when the transistor Q.sub.1 is rendered inoperative. Connected to a secondary winding N.sub.2 of the transformer T is a rectifying--smoothing circuit comprising a diode D.sub.1 and capacitor C.sub.1, whereby a DC output voltage is applied to a load 3.
When the transistor Q.sub.1 is turned on, a current is caused to flow from the DC power source connected to the input terminal 1 to the primary winding N.sub.1 of the transformer T, and resultant power is stored therein in the form of magnetic energy. Subsequently, when the transistor Q.sub.1 is turned off, the magnetic energy stored in the primary winding N.sub.1 is caused to appear, in the form of a counterelectromotive force, at the secondary winding N.sub.2, thus resulting in the diode D.sub.1 being rendered operative so that power is supplied to the load 3.
Output voltage, divided by resistors R.sub.3 and R.sub.4, is derived and compared in an error amplifier 4 with a reference voltage available from a reference voltage source 5. As a result, a feedback voltage corresponding to the difference between the output voltage and the reference voltage is applied from the error amplifier 4 to the pulse-width controlling circuit 2. With the pulse-width controlling circuit 2, the pulse-width of a drive pulse to turn on the transistor Q.sub.2 is determined on the basis of the feedback voltage derived from the error amplifier 4, and the drive pulse is imparted to the base of the transistor Q.sub.1 through the resistor R.sub.1. More specifically, when the output voltage decreases, the feed voltage also decreases so that drive pulse to increase the "on" period of the transistor Q.sub.1 is imparted to the base thereof. Conversely, when the output voltage increases, the feedback voltage also builds up, so that a drive pulse to decrease the "on" period of the transistor Q.sub.1 is derived from a voltage comparator 17.
It is usual that the design is made such that only the cutoff and saturation regions of the switching transistor Q.sub.1 are utilized to minimize loss which tends to occur during the switching operation. To make the transistor Q.sub.1 saturated, it is necessary to flow a sufficient base current to establish a so-called "over-drive" condition.
Let it be now assumed that the potential of the pulse derived from the pulse-width controlling circuit 2, i.e., potential occurring at a point A in FIG. 1, is at low level (i.e., nearly equal to 0 V) with respect to the input voltage V.sub.i. Under such a condition, the transistor Q.sub.1 is turned on because of the base thereof being positively biased, so that a base current I.sub.b1 flows through the resistor R.sub.1. The values for the resistors R.sub.1 and R.sub.2 are determined such that there flows such a sufficiently high base current I.sub.b1 to establish the over-drive condition of the transistor Q.sub.1. As a result, a large quantity of charges are stored between the base and the emitter of the transistor Q.sub.1. Thus, even when the potential at the point A is changed to high level (nearly equal to V.sub.i), a period of time will be taken before the transistor Q.sub.1 which has been conducting, is completely turned off. Collector current I.sub.c1 decreases gradually during the interval between when the transistor Q.sub.1 begins to be driven toward "off" state and when it is completely turned off, and thus a counterelectromotive force begins to occur at the transformer T. Consequently, as shown in FIG. 2(a), voltage V.sub.CE between the collector and the emitter of the transistor Q.sub.1 varies with time, so that there is a period of time .DELTA.t that the collector-emitter voltage V.sub.CE and the collector current I.sub.c1 occurs at the same time. The product of the voltage and current which occurs during the period of time .DELTA.t is termed "turn-off loss", which accounts for a large proportion of the total loss of the transistor Q.sub.1.
With the conventional circuit arrangement of FIG. 1, to reduce such turn-off loss by reducing .DELTA.t, it is only required that the value for the resistor R.sub.2 be reduced so as to permit of quick discharge of charges accumulated between the base and the emitter of the transistor Q.sub.1. However, there is a tendency that the base bias voltage of the transistor Q.sub.1 is changed if the value for R.sub.1 alone is reduced; thus, the value for the resistor R.sub.1 should also be reduced Disadvantageously, however, if both the value for the resistor R.sub.1 and that for the resistor R.sub.2 are reduced, then the base current I.sub.b1 of the transistor Q.sub.1 will be further increased. As known in the art, the base current I.sub.b1 is a current needed to turn on the transistor Q.sub.1 and does not contribute directly to conversion efficiency. Thus, the increase in the base current I.sub.b1 will lead to a decrease in the efficiency. As will be seen from the foregoing discussion, the prior-art arrangement is disadvantageous in that a certain amount of turn-off loss is inevitably caused since an attempt to reduce the value for the resistor R.sub.1 has its limit.